The present invention relates generally to X-ray tubes, and more particularly to an X-ray tube having a higher ratio of X-ray energy flux to power deposited in the target.
X-ray devices used in the medical field contain an X-ray tube which typically includes a cathode which is heated to emit a beam of electrons, a (typically rotating) anode having a target with a surface facing the cathode, and a surrounding glass and/or metal frame containing an X-ray-transparent window secured by a window mount. Typically, the cathode is oriented such that the electrons strike a focal spot on the target surface at an angle which is generally ninety degrees with respect to the target surface. Some emitted electrons strike the target surface and produce X-rays, and some of the X-rays exit the frame as an X-ray beam through the X-ray-transparent window. Typically, the X-ray window is positioned such that it receives X-rays which leave the target surface at an angle of generally seven degrees with respect to the target surface. Some emitted electrons do not produce X-rays and may be backscattered when they strike the target surface. Many of the backscattered electrons go on to strike and heat the frame including the X-ray-transparent window and the window mount. The frame is also heated from within by other sources such as thermal radiation. The heated frame is typically cooled by a liquid coolant, such as oil or water, located between the frame and a surrounding casing having its own X-ray-transparent window.
Generally less than one percent of the power of the electrons striking the target surface is converted into X-ray power. Increasing the power of the electron beam will increase the X-ray power output of the tube. However, increasing the power of the electron beam leads to unacceptably high thermal loading of the target which ultimately limits the X-ray power output. What is needed is an X-ray tube assembly, and a method for producing X-rays, which increases the ratio of X-ray tube power per target thermal load.
In a first expression of an embodiment of the invention, an X-ray tube assembly includes an X-ray tube anode, an X-ray tube cathode, and an X-ray tube window. The anode includes an X-ray-producing target having a surface. The cathode has an electron-beam axis. The electron-beam axis intersects the target surface at a focal point, and the electron-beam axis is oriented at a first angle with respect to the surface of the target. The first angle is between and including fifteen degrees and sixty degrees. The window includes a surface having a center point, and a line between the focal and center points makes a second angle with respect to the target surface.
In a second expression of an embodiment of the invention, an X-ray tube assembly includes an X-ray tube anode, an X-ray tube cathode, and an X-ray tube window. The anode includes an X-ray-producing target having a surface. The cathode has an electron-beam axis. The electron-beam axis intersects the target surface at a focal point, and the electron-beam axis is oriented at a first angle with respect to the surface of the target. The first angle is between and including fifteen degrees and sixty degrees. The X-ray tube cathode produces electrons which strike the target producing X-rays having energies less than generally two hundred kilovolts. The window includes a surface having a center point, and a line between the focal and center points makes a second angle with respect to the target surface. The second angle is less than the first angle. The electron-beam axis and the center point define a plane which is oriented generally perpendicular to the target surface.
A first method of the invention is for producing X-rays and includes steps a) through c). Step a) includes generating a beam of electrons, wherein the beam has an electron-beam axis. Step b) includes orienting the beam of electrons to strike a focal spot on a surface of an X-ray-producing target to generate X-rays such that the electron-beam axis makes a first angle with respect to the surface of the X-ray target and such that the first angle is between and including fifteen degrees and sixty degrees. Step c) includes utilizing those X-rays which make a second angle with respect to the surface of the target.
A second method of the invention is for producing X-rays and includes steps a) through c). Step a) includes generating a beam of electrons, wherein the beam has an electron-beam axis. Step b) includes orienting the beam of electrons to strike a focal spot on a surface of an X-ray-producing target to generate X-rays having energies less than generally two hundred kilovolts such that the electron-beam axis makes a first angle with respect to the surface of the X-ray target and such that the first angle is between and including fifteen degrees and sixty degrees. Step c) includes utilizing those X-rays which make a second angle with respect to the surface of the target, wherein the second angle is less than the first angle, and which, together with the electron-beam axis, define a plane oriented generally perpendicular to the surface of the target.
Several benefits and advantages are derived from choosing the first angle (which typically is called the electron-beam incident angle and referred to as xe2x80x9calphaxe2x80x9d) and the second angle (which typically is called the X-ray emission angle and referred to as xe2x80x9cbetaxe2x80x9d) in accordance with the invention. For example, computer simulations benchmarked by experimental data show an X-ray energy flux enhancement of generally 1.5 when beta equals seven degrees and when alpha equals fifteen to twenty degrees. The enhancement is computed in comparison to the X-ray energy flux of the prior art design wherein beta is seven degrees and alpha is ninety degrees, wherein the deposited power (i.e., the thermal load measured by temperature) and focal-spot temperature in the target is the same in the inventive and prior-art designs, and wherein the X-ray spectra of the inventive design is filtered to obtain the same mean photon (i.e., X-ray) energy as that of the prior-art design, for proper comparison, as can be appreciated by those skilled in the art. An enhancement of 1.5 means a fifty percent increase in X-ray power output for the same thermal load and focal-spot temperature in the target for the inventive design compared to the prior-art design. It also means the X-ray tube of the inventive design can be operated at the same X-ray power output, but at a lower temperature (to increase tube life) compared to the X-ray tube of the prior-art design.